German Published Patent Specification No. 2,650,533 claims a process for the preparation of carbonic acid aryl esters of polyalkylene oxide-diols, lengthened via carbonate groups, from polyalkylene oxide-diols with a Mn above about 135, preferably above about 800, and carbonic acid bis-aryl esters, which is characterized in that polyalkylene oxide-diols with molecular weights Mn (number-average) above about 135 are heated together with carbonic acid bis-aryl esters, preferably at temperatures between about 100.degree. C. and 200.degree. C., in vacuo below about 35 mm Hg in the presence of catalysts, less than one mol of carbonic acid bis-aryl ester being employed per OH group, and the hydroxyaryl compound formed is distilled off.
Suitable polyalkylene oxide-diols are, in particular, those of the formula I ##STR1## wherein
R' and R" independently of one another are H or C.sub.1 -C.sub.4 -alkyl,
a is an integer from about 1 to 6 and
b is an integer from about 3 to 350, in particular from about 3 to 250.